とんでもございません

Commonly considered a mistake because とんでもない is handled as an adjective in modern Japanese grammar and replacing the ない with ございません to make a polite form is thus a mistake according to the grammar. Although etymologically とんでもない is not a single word but an expression (途でもない), it is used like a single word today except for the non-standard polite forms とんでもありません and とんでもございません.